The membrane glycolipids and glycoproteins of tumorigenic and nontumorigenic mouse mammary cells were compared. The cells studied include (1) cell lines derived from mouse mammary carcinomas of varying etiologies, (2) a series of clonal cell lines (DMBA/BALB) which are nontumorigenic at lower passage levels and tumorigenic at high passage levels and (3) primary cells derived from normal mammary glands and tumors produced by transplantation of the cultured mammary cells. The tumorigenic and nontumorigenic mammary cells were distinguishable with regard to their trypsin-sensitive, surface fucopeptides. The tumorigenic cells were enriched in the larger fucopeptides, as has been observed with tumorigenic fibroblasts. The size distribution of the trypsin-sensitive, surface fucopeptides of the tumorigenic mammary cells was largely independent of cell population. Another series of fucosylated components, namely the O-glycosylated amino acid compounds, FL3 and FL4, also differ in tumorigenic and nontumorigenic mammary cells. The more complex component, FL4, was reduced in the tumorigenic cells. External proteins were analyzed by lactoperoxidase catalyzed iodination and glycoproteins by metabolic labeling with 3H galactose and 3H fucose. Although some glycoprotein differences among the cell lines were found, e.g., with respect to a large, external glycoprotein, no consistent difference between the nontumorigenic and tumorigenic cells was observed. The tumorigenic DMBA/BALB cells were generally found to have a smaller proportion of the more complex gangliosides as compared to the nontumorigenic DMBA/BALB cells. Likewise, mammary tumors and preneoplastic tissue were reduced in the more complex gangliosides, as compared with normal mammary glands.